وَقَدْ خَابَ مَن دَسَّىٰهَا
Sentence structure
وَقَدْ coordinates and confirms خَابَ. مَن is its subject; دَسَّىٰهَا is the relative clause with understood subject and attached object هَا.
Ayah summary
And unsuccessful is the one who buries his soul beneath its desires.
وَقَدْ — And indeed
Grammar: وَ coordinates and قَدْ confirms the following perfect.
خَابَ — he fails
Grammar: A third-person masculine-singular perfect whose explicit subject is مَن.
مَن — who
Grammar: A relative noun functioning as the nominative subject of خَابَ.
دَسَّىٰهَا — buries it
Grammar: A Form II third-person masculine-singular perfect in the relative clause; subject is understood and attached هَا is object.